Ionization potentials and proton affinities

The four major quantities on which we will focus are atomization energies, electro afiinities, ionization potentials and proton affinities. We ll look at each of them i turn in this section. Note that all of them are conventionally defined to be greater tha... 

The semi-empirical methods have better MAD s than th Hartree-Fock-based methods, indicating that their parametrization ha accounted for some of the effects of electron correlation. However, thei maximum errors are very large. Semi-empirical methods are especiall poor at predicting ionization potentials and proton affinities. 

The F and R parameters are qualitatively analogous to the field and resonance parameters, 5 and SR, of Swain and Lupton19 that is, they measure the a and 7r-electronegativities, respectively, of substituents. However, the F and R values are more appropriate for correlating processes in which a localized positive charge develops than are the S and 91 values. Hence the F and R values correlate lone pair ionization potentials and proton affinities better than the corresponding and 91 values do. 

W1/W2 theory and their variants would appear to represent a valuable addition to the computational chemist s toolbox, both for applications that require high-accuracy energetics for small molecules and as a potential source of parameterization data for more approximate methods. The extra cost of W2 theory (compared to W1 theory) does appear to translate into better results for heats of formation and electron affinities, but does not appear to be justified for ionization potentials and proton affinities, for which the W1 approach yields basically converged results. Explicit calculation of anharmonic zero-point energies (as opposed to scaling of harmonic ones) does lead to a further improvement in the quality of W2 heats of formation at the W1 level, the improvement is not sufficiently noticeable to justify the extra expense and difficulty. 

Selective synthesis of 2-methyl naphthalene has been studied over HZSM-5, HZSM-11, HSABO-11, HZBS-11, Zn-ZSM-11, Ga-ZSM-11, HY and HZSM-20 type zeolites. The nature of the naphthalene-methanol interaction has been investigated in order to elucidate the reaction mechanism. According to the data obtained by FT-IR, TPD of naphthalene, ionization potential and proton affinity of different aromatic rings, zeolites with medium pores and with sites of medium or high acid strength are necessary for the reaction. The results seem to be consistent with the Rideal type mechanism. 

W. A. Chupka and J. Berkowitz, Photoionization of methane ionization potential and proton affinity of CH4, J. Chem. Phys. 54, 4256-4259 (1971). 

One of these techniques, referred to as Gaussian 2 (G2) theory, is described in this article. G2 theory is a general procedure, based on ab initio molecular orbital theory, for the accurate prediction of energies of molecular systems. This method has been widely used for the calculation of bond energies, enthalpies of formation, ionization potentials, electron affinities, ionization potentials, and proton affinities. The status of G2 theory, some modifications of the theory, and examples of its applications are presented in this article. 

TABLE 8.1. Ionization Potentials (IP), Proton Affinities (PA), Values, and BK3 Affinities... 

Electrons are selectively captured by the pyrimidines and the holes formed on the base stack, or transferred to the base stack, are nearly all trapped by Gua, giving Gua +. This species is in a reversible equilibrium with its deprotonated form Gua(NI-H) (Fig. 2). While initial distributions between the pyrimidines are nearly equal upon annealing, or irradiation to high dose, the excess electron transfers to cytosine as the most stable site as a result of protonation of Cyt at N3 forming Cyt(N3+H) [10]. Molecular orbital calculations show that the major factor that affects the relative yields of DNA base ion radicals is the difference in both the ionization potential and electron affinity ofthe DNA bases [11]. In addition, experimental and theoretical work shows that variations in relative radical yields are also affected by the... 

A similar type of research took place also in nuclear physics during the thirties with a systematic characterization of different properties for a number of atomic nuclei [24]. As an example can be mentioned the studies of the neutron cross sections as a function of the number of neutrons or protons in the nuclei, which showed systematic variations with very small values at certain numbers corresponding to nuclei with 20, 50, 82 and 126 neutrons. This discovered periodicity was rather different compared with the periodicity of atomic properties as the first ionization potential and electron affinity for alkali and noble gas atoms. Speaking at a meeting of the Chemical Society on April 19, 1934, the centenary of the birth of Mendeleev, Rutherford concluded, /< may be that a Mendeleev of the future may address the Fellows of this Society on the Natural Order of Atomic Nuclei and history may repeat itself [25]. Measurements of for example nuclear spins for a number of isotopes also showed a similar type of periodicity as found in neutron cross sections. This kind of periodicity could not at that time be understood from the commonly used liquid drop model [26] but based on the single particle model formulated by Mayer, Haxel, Jensen and Suess in 1949 [27]. 

Figure 2 is a plot of the ionization potential against proton affinity for a number of neutral molecules, where again the donor atom is C, N, O, or F. The molecules include HF, H20, NH3, N2, CO, singlet CH2, and a sampling of ethers, alcohols, amines, esters, amides, nitriles, and other organic molecules. The latter compounds are considered to represent several hundred such molecules for which both IP and PA are known (20-21). The straight line drawn has a slope of —0.55, and the correlation coefficient is 0.939. 

Determination of proton affinities (basicities) and acidities in the gas phase provides a means of systematically representing a large number of ion-molecule reactions and of the relationships between these quantities and bond strengths, ionization potentials and electron affinities without disturbance by solvation phenomena 47). 

Properties calculated for the species include ionization energies and proton affinities [15], dipole moments and polarizibility functions [16], and electron affinities [17]. The ground state reduced potential curve obtained from measured spectra has been analyzed [18], and bond critical points and several electronic properties have been calculated for a series of molecules including BH using GAUSSIAN 80 [19]. 

The original paper defining the Gaussian-2 method by Curtiss, Raghavachari, Trucks and Pople tested the method s effectiveness by comparing its results to experimental thermochemical data for a set of 125 calculations 55 atomization energies, 38 ionization potentials, 25 electron affinities and 7 proton affinities. All compounds included only first and second-row heavy atoms. The specific calculations chosen were selected because of the availability of high accuracy experimental values for these thermochemical quantities. 

In this equation, r) the absolute hardness, is one-half the difference between /, the ionization potential, and A, the electron affinity. The softness, a, is the reciprocal of T]. Values of t) for some molecules and ions are given in Table 8.4. Note that the proton, which is involved in all Brdnsted acid-base reactions, is the hardest acid listed, with t — c (it has no ionization potential). The above equation cannot be applied to anions, because electron affinities cannot be measured for them. Instead, the assumption is made that t) for an anion X is the same as that for the radical Other methods are also needed to apply the treatment to polyatomic... 

Measuring physical-chemical properties of the clusters, such as ionization potential (IP), binding energy (BE), electron (EA) and proton affinity (PA), fragmentation channels, electronic structure and so on, provides a basis for the comprehension of the intrinsic forces acting in the clusters and governing their dynamics. Theoretical computation of these quantities may provide a feedback to evaluate the quality of the calculations and the accuracy of the experimental determinations. 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

TABLE 19. Nitrile proton affinities, adiabatic ionization potential and homolytic bond dissociation energies0... 

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